44                                    PHYSICS OF THE AIR
of temperature with increase of height to an approximately isothermal condition. Normally, however, the temperature gradient changes from a rapid to practically a zero decrease of temperature with increase of height, much more abruptly than one would infer from the given curves. But this more or less abrupt change varies considerably in altitude from day to day. Therefore, when a large number of actual gradients are averaged an apparent gradual transition is indicated.
Two other features calling for some attention are shown by the lower portion of the summer gradient; namely, the fact that through the first }4 kilometer the temperature decreases but slowly, and the further fact that through the second J^ kilometer it decreases more rapidly than anywhere else, short of very considerable altitudes. Now nearly all the observations from which this summer gradient was constructed were obtained during the early forenoon. Hence the average slow decrease of temperature with increase of elevation is the result only of ordinary morning inversions. On the other hand, the rapid decrease of temperature through the second % kilometer is expressive of the adiabatic gradient of unsaturated air that commonly exists during summer afternoons up to a level of at least 1 kilometer. In this case it has persisted ipper portion throughout the night, and been modified, as explained, iperature inversions only in its lower half.
Atmospheric Stratification. — Almost every individual sounding of the atmosphere shows more or less stratification — layer upon layer of air differing more or less from each other in lapse rate, humidity, and direction and velocity of movement. The warmer an upper layer the less likely it is to be penetrated by convection from below — a cooling pro cess — or itself to fall to a lower level — a warming process. The first would put relatively dense air in light, and the second light air in dense, an unstable situation in either case. In many cases convection is arrested by one of these layers as by a physical ceiling.
Not only is the atmosphere generally stable in the respect of one layer to another, but also within the individual layers, especially when they are free from cloud, and the less the lapse rate the greater this stability. An obvious measure of this stability would be the force of restitution per unit mass of air per unit vertical displacement; but this force1 is directly proportional to the difference between the densities of the displaced and the then surrounding air. In symbols,
a - b
where a is the adiabatic, and b the current, lapse rate, and T the absolute temperature of the surrounding air at the final position.
Why the Temperature of the Atmosphere Decreases with Increase of Height.— It is not, perhaps, obvious why the temperature of the -atmos-